The present invention relates to a system for controlling the air-fuel ratio for an internal combustion engine emission control system which comprises a three-way catalyst, and more particularly to a system for controlling the air-fuel ratio to a value approximating the stoichiometric air-fuel ratio so as to effectively operate the three-way catalyst.
Such a system is a feedback control system, in which an O.sub.2 sensor is provided to detect the oxygen content of the exhaust gases to generate an electrical signal as an indication of the air-fuel ratio of the air-fuel mixture supplied by a carburetor. The control system comprises a comparator for comparing the output signal of the oxygen sensor with a predetermined value, an integrating circuit connected to the comparator, a driving circuit for producing square wave pulses from the output signal of the integrating circuit, and an on-off type electromagnetic valve for correcting the air-fuel ratio of the mixture. The control system operates to determine whether the feedback signal from the O.sub.2 sensor is higher or lower than a predetermined reference value corresponding to the stoichiometric air-fuel ratio for producing an error signal for actuating the on-off type electromagnetic valve to thereby control the air-fuel ratio of the mixture.
In the conventional system, as shown in FIGS. 1a and 1b, the output waveform P.sub.1 of the O.sub.2 sensor varies from a maximum output voltage thereof to a minimum one, because the O.sub.2 concentration in the exhaust gases exceeds values corresponding to the maximum and minimum outputs due to the control delay of the control system. As shown in FIG. 1a, the output voltage of the O.sub.2 sensor varies steeply at a reference voltage V.sub.R which corresponds to the output voltage caused by exhaust gases when a mixture of the stoichiometric air-fuel ratio (St) is supplied to the engine and burned. Therefore, it may be regarded that a middle value M between the maximum and minimum values in each cycle of the output waveform of the O.sub.2 sensor is constant and substantially equal to the voltage V.sub.R corresponding to the stoichiometric air-fuel ratio. Thus, in the conventional system, the middle value M is set as the reference value of the comparator for comparing the air-fuel ratio of the mixture supplied to the engine.
On the other hand, copending Patent application Ser. No. 174,385, which was assigned to the same assignee as this patent application, discloses a system intended for improvement of the control delay in such a conventional system, in which the on-off electromagnetic valve is operated by a dither signal having a high frequency and a small amplitude. However, it is not proper to use the middle value of each cycle in the output waveform P.sub.2 of the O.sub.2 sensor dependent on the dither signal D as a reference value, because the middle value of the output waveform of the O.sub.2 sensor does not always coincide with the reference voltage V.sub.R corresponding to the stoichiometric value, as shown in FIG. 1b. Therefore, the middle value cannot be used as a reference value.